Integrated circuits ("ICs") have many uses in industries ranging from communications to consumer electronics. By way of example, a power transistor IC is formed by fabricating one or more transistor cells on a silicon wafer, commonly referred to as a transistor "chip". The transistor chip is attached to an isolating layer, normally a ceramic substrate, which is thermally, but not electrically, conductive. The ceramic substrate is itself attached to a thermally conductive mounting flange. A protective cover is secured to the flange, covering the substrate and transistor chip, thereby forming a power transistor IC "package."
Various electrically conductive (e.g., thin metal) leads may be attached to, and extend away from the package, in order to connect common terminals of the transistor chip to other circuit elements located, e.g., on an adjacent printed circuit ("PC") board. For example, with a bipolar junction type power transistor, respective electrical leads attached to the package are connected to a base, emitter and collector of the transistor chip.
Because the power transistor package generates a significant amount of heat during operation, the bottom surface of the package mounting flange is normally directly secured to a metallic heat sink underlying the PC board. For example, a single layer PC board has a layer of dielectric material between respective top and bottom conductive surfaces, wherein the bottom surface acts as a reference ground. This bottom surface is connected, usually with screws or solder, to the underlying metal heat sink, so that the bottom surface and the heat sink have the same ground potential with respect to any circuit elements attached to the top surface of the PC board.
There are several known techniques for securing an IC package to a heat sink surface. For example, as illustrated in FIG. 1, an exemplary IC package 20 may be secured to a heat sink 22 by a solder connection 28 between the bottom surface of the package mounting flange 26 and the surface of the heat sink 22.
While this approach is relatively simple, the solder weld material 28 will invariably have a different thermal expansion coefficient than the respective (typically metal) mounting flange 26 and heat sink 22. As a result, the bond between the mounting flange 26 and heat sink 22 will weaken or even be destroyed by the thermal expansion stress between the respective layers, especially when subjected to repeated changes in temperature during each use of the IC package 20. Further, the presence of the intervening bonding material layer 28 may lesson the effectiveness of the heat conduction between the flange 26 and heat sink 22. A still further disadvantage with this approach is that, in order to remove the IC package 20 for repair or replacement, the entire heat sink 22 must be heated to break the solder bond 28, thereby causing any other solder bonds on the same heat sink 22 to be weakened.
Referring to FIG. 2, as an alternative to using a solder connection, the IC package 20 may be secured to the heat sink 22 with a pair of screws 24 through openings located on respective ends of the mounting flange 26. Referring to FIGS. 3 and 4, still another technique for securing an IC package to a heat sink is to insert one or more screws 30 into, so as to be protruding above, the surface of the heat sink 22. A resilient metal strip 32 is extended from the screw(s) 30 and is shaped so as to apply a clamping force upon the cover of the IC package 20, thereby distributing a substantially centered force that "secures" the mounting flange 26 against the heat sink 22.
Still another method for securing an IC package to a heat sink is disclosed and described in now U.S. Pat. No. 5,869,897, "Mounting Arrangement For Securing An Integrated Circuit Package To A Heat Sink," which is fully incorporated herein by reference for all it teaches. As taught therein, and as illustrated in FIG. 5, a top surface 52 of a protective cover 50 of an IC component package 40 is provided with a centered-protrusion 54. A resilient retaining-spring 46 formed into a ribbon-like shape having opposing ends 56 and 58 that extend from a curvelinear bottom surface 60 is provided with an opening 62 sized to mate with the centered protrusion 54.
To mount the IC package 40 to a heat sink 42, the retaining-spring opening 62 is compressively mated onto the package cover protrusion 54 as the mounting flange 45 of the IC package 40 is inserted between substantially parallel walls 44 and 48 protruding from the heat sink 42, such that the opposing retaining-spring ends 56 and 58 extend away from the package cover 50 at substantially the same, albeit reverse angles. The walls 44 and 48 are distanced from each other just so as to cause moderate compression of the opposing retaining-spring ends 56 and 58 toward each other as the flange 45 is inserted against the heat sink 42.
The walls 44 and 48 are each provided with a respective plurality of notches 64 and 68, which extend substantially parallel to the heat sink 42 in a "ratchet-type" relief pattern. Once the mounting flange 45 is pressed against the heat sink 42, the opposing spring ends 56 and 58 are retained in place by the respective wall notches 64 and 68. In this manner, the spring 46 applies a retaining force against the package cover 50, thereby securing the mounting flange 45 against the heat sink 42, as indicated by the arrow 70.
With any of the above-illustrated methods for securing an IC package to a heat sink, once the IC package is secured to the heat sink, electrical leads extending from the package (not shown in FIGS. 1-5) must be connected to respective conductive surface leads or areas, e.g., located on an adjacent PC board attached to the heat sink.
By way of illustration, referring to FIG. 6, the mounting flange 86 of an IC package 80 is mounted on a heat sink 82 via a conventional solder weld 84. A single layer PC board 88 is also secured to the heat sink 82, e.g., by screws (not shown) adjacent both sides of the package 80. The PC board includes a metal top surface 90, a layer of dielectric material 92, and a metal bottom surface 94, respectively, wherein the bottom surface 94 and attached heat sink 82 collectively act as a reference ground with respect to circuit elements (not shown) attached to the top surface of the PC board 88. Respective leads 96 and 98 extend from opposite sides of the package 80 and are connected to corresponding conductive paths formed on the top surface 90 of the PC board 88 via respective solder welds 100 and 102.
As with the problems of using a solder weld connection between the respective package flange (28) and heat sink (22) described above in conjunction with FIG. 1, the solder weld connections 100 and 102 are also prone to problems caused by different thermal expansion coefficients between the solder material, the conductive surface 90, and the respective (metal) leads 96 and 98. In particular, the solder material can crystallize after repeated heating and cooling, causing welds 100 and 102 to weaken and/or fail, with the respective leads 96 and 98 lifting and separating from the surface 90 of the PC board 88.
Thus, it would be desirable to provide improved arrangements for securing an IC component package to a heat sink, whereby solder-welds are eliminated.